Spectroscopy is one of the primary methods of characterizing molecules, materials, pharmaceuticals, or even devices. The simplest spectroscopic instruments measure one dimensional spectral data, e.g. ultraviolet (UV)-visible (Vis) spectroscopy for electronic transitions, Fourier transform infrared spectroscopy (FTIR) spectroscopy for vibrational transitions, and nuclear magnetic resonance (NMR) spectroscopy for nuclear transitions. Based upon the maturity of these simple techniques commercial instrumentation systems are available from several vendors which may provide in addition to the spectral data acquisition supporting features such as automated sample handling, spectral signal analysis, and material identification for example. However, all of these approaches suffer a common drawback that the resulting spectra from a sample may be so congested as to render the desired “finger-printing” of spectral characteristics to identify components of the sample impossible
To circumvent this spectral congestion problem within NMR two-dimensional NMR was proposed by Professor J. Jeener in 1971 and first implemented in 1976 at the Université Libre de Bruxelles, in 1971. This experiment was later implemented by Aue et al in “Two-Dimensional Spectroscopy—Application to Nuclear Magnetic Resonance” (J. Chem. Phys., Vol. 64, pp. 2229-2246). In these measurements the spectrum has an excitation and an emission axis, giving a two-dimensional (2D) spectrum which reveals the information formerly hidden in one-dimensional (1D) spectroscopy such as depicted in FIG. 1A. In each graph the diagonal would corresponds to observing the two emission peaks within a 1D spectrum. However, in the 2D spectrum one plots the excitation and the emission axes, along with the time at which the 2D spectrum is obtained. Hence one can directly observe dynamics from one state to the next, even when the 1D spectrum shows no features due to congestion. In the intervening 35 years 2D-NMR spectrometers have gone from proof-of-concept to being standard commercial instruments offered alongside commercial 1D spectrometers. Also depicted in FIG. 1B is a 2D spectrum for excitons within a semiconductor quantum well with the excitonic levels depicted in FIG. 1C whilst the associated time difference between excitation and emission is depicted in FIG. 1D.
Now considering optical spectrometry commercial and research 1D spectrometers reveal a static in time spectrum of the system being characterized whereas it is clear that systems evolve with time, such as for example wherein excitations are subsequently followed by emissions at times dependent upon the lifetime of the excited state. In some instances the excitation (optical pump) is followed by a second optical signal (optical probe) whereas in others it is not. In the former the optical probe stimulates emission whereas in the latter it is spontaneous. Hence it would be beneficial to also provide time resolved 1D spectral data within the deep ultraviolet (DUV), UV-Vis, and infrared (IR), both near infrared (NIR) and far infrared (FIR). Such time resolved 1D spectra are obtained via systems employing pulsed optical pump/optical probe spectroscopy where the time at which the probe pulse measures the 1D spectrum is dictated by the time interval between the optical pump and optical probe pulses. This approach has been developed since the 1970s and is routinely used by thousands of researchers worldwide and as the technology and methods of time resolved 1D spectroscopy have matured then commercial time resolved 1D UV-Vis spectrometers have become available. However, the spectral congestion problem still remains for FIR, NIR, UV-Vis or DUV spectroscopy both direct and Fourier transform.
Circa 2000, the ultrafast laser spectroscopy community developed an optical analog of 2D-NMR at both IR and UV-Vis energies allowing characterization of vibration and electronic transitions in materials respectively. Whilst the system developed is a powerful solution to this spectroscopic problem it is also enormously complex so that whilst these time resolved 2D IR and 2D UV-Vis spectroscopy measurements have yielded remarkable results for the small number of pioneering researchers, the methods themselves are so complex that globally only about a dozen groups have establish such methods. Accordingly it would be beneficial to provide the global community with a commercial solution to 2D IR and 2D UV-Vis spectroscopy. Accordingly, to do so requires a fundamental reconsideration of the technique given the complexity of the current research systems.
Amongst the global research groups within the field are Keith Nelson Group at Massachusetts Institute of Technology and the Graham Fleming Group at University of California, Berkeley. Considering the instrumentation of these groups then we find the following instrumentation solutions as depicted in respect of FIGS. 2A and 2B.
FIG. 2A depicts the two-dimensional Fourier Transform optical spectroscopy (FTOPT) system of the Keith Nelson Group (http://nelson.mit.edu/index.php?option=com_content&view=article&id=27&Itemid=56). Accordingly as depicted a laser pulse first enters the beam shaper (blue box), consisting of a spatial light modular (SLM 1) at the focus of a telescope. A phase pattern is applied that generates the desired spatial beam geometry wherein the beams then enter the pulse shaper (green box). A telescope first inverts the beams, which then impinge upon a grating G, spectrally dispersing the beams. A cylindrical lens CL focuses the spectrum onto SLM 2. The phase pattern applied in the horizontal dimension controls the temporal (spectral) phase and the sawtooth pattern in the vertical dimension diffracts the beams (d). The diffracted beams return through the pulse-shaping apparatus, hitting a pick off minor (M) that sends the beams to the sample. The signal is generated in a phase-matched direction given by the specified beam geometry. The full amplitude and phase of the emitted signal is retrieved via heterodyne detection and spectral interferometry.
For the Graham Fleming Group, their techniques include 2D Electronic Spectroscopy as depicted in FIG. 2B (http://www.cchem.berkeley.edu/grfgrp/pages/Techniques/2D.html), wherein the ultrafast optical source is split into four optical beams which are then individually set in polarization, and Pump Probe Spectroscopy as depicted in FIG. 2C (http://www.cchem.berkeley.edu/grfgrp/pages/Techniques/Pump_Probe.html) wherein the optical system allows pumping with 800 nm, 400 nm, visible, and near-infrared light. The 800/400 nm pump line is produced by the fundamental or second harmonic of a regenerative amplifier and visible and near-infrared light is generated from an optical parametric amplifier (OPA). As illustrated the OPA is a two-pass OPA creating near-infrared signal and idler beams from an 800 nm input beam which are then overlapped in a difference frequency generating crystal (e.g. AgGaS). The pump and probe are overlapped onto the sample and changes in the intensity of the probe beam are detected at an imposed frequency created using an optical chopper and measured using a lock-in amplifier technique. These changes are monitored as a function of time, and a reference mid-IR beam passes through the sample at a separate location to allow us to subtract general fluctuations in the laser beam.
Also within the prior art are solutions to 2D optical spectroscopy including World Patent Application WO/2009/143957 and corresponding US Patent Application 2011/0,141,467 by Brixner et. al. from the University of Wurzburg entitled “Device and Method for Coherent Multi-Dimensional Optical Spectroscopy.” In this approach, as depicted in FIG. 3A, a single laser source is combined with a series of beam splitters, lenses and collimators and is based upon a design originating from the lead inventors post-doctoral work with the Graham Fleming group at University of California—Berkeley. Due to the design approach dozens of vibration susceptible optics are employed as common with the design from the University of California—Berkeley. World Patent Application WO/2009/075702 and corresponding US Patent Application 2010/0,171,952 by Deflores et al entitled “Two-Dimensional Fourier Transform Spectrometer” from the Massachusetts Institute of Technology (MIT) wherein, as depicted in FIG. 3B, three optical signals are employed to resolve the collinear signals rather than the original approach of using four separate signals (a fifth signal is also used to assist in alignment). This three signal approach is correspondingly complex from a hardware design and integration perspective.
U.S. Pat. No. 7,760,342 by Zanni et. al entitled “Multi-Dimensional Spectrometer” from University of Wisconsin, as depicted in FIG. 3C, employs a single pulse shaper to perform “one quantum” 2D experiments in a non-collinear geometry wherein the pump pulse profile, typically comprising two or more sub-pulses, and the probe pulse are incident to the sample. Whilst the sub-pulses contact the sample of interest collinearly, having followed the same optical path, the probe pulse is offset at an angle such that the sub-pulses and probe pulse only intersect at the sample. Examples of pulse shapers taught by Zanni include acousto-optic modulators, spatial light modulators, and digital micro-mirror devices in conjunction with diffraction gratings, folding optics, beam splitters etc. Accordingly the resulting optical assembly still comprises a large number of optical elements requiring alignment and isolation for vibration etc.
In contrast the inventors have established an approach exploiting a monolithic platform which provides simplicity and robustness lacking in the prior art enabling the invention to be packaged into a simple “black-box” module for commercial deployment in 2D optical spectroscopy instruments. The invention exploits a dual pulse shaper approach that uniquely enables polarization shaping of the complete electric field without any moving parts. This approach enables both one quantum and two quantum signals, polarization switching to probe more systems, and a completely co-linear beam geometry that enables simpler design.
Accordingly the inventors have developed an approach to 2D optical spectroscopy that eliminates the substantial number of optical elements and associated complexity and significant number of scientists currently needed to actually perform these measurements. Initial preliminary tests presented at FEMTO10—Madrid Conference on Femtochemistry (July 2011), 18th International Conference on Ultrafast Phenomena (July 2012) and to be presented at the Ultrafast Phenomena conference (2012) and the American Chemical Society Fall National Meeting (August 2012) have shown that simple approach is up to four times more stable than the complex instruments of the two leading research groups, namely Keith Nelson Group at Massachusetts Institute of Technology and the Graham Fleming Group at University of California, Berkeley. Beneficially the measurement system developed by the author is solid state, thereby removing all moving parts, but also uniquely provides for polarization shaping of the optical pulses thereby giving rise to further improvements in performance and functionality.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.